As of 14 July 2009, Oracle is now introducing a new method for patching, patch set updates, or PSU. According to MOSC notes 854428.1 and 850471.1, Oracle PSU is a new patching strategy whereby the DBA can choose only "recommended" and "proactive" patches, instead of all of the patches in a quarterly Critical Patch Update (CPU).
An Oracle PSU contains recommended bug fixes and "proactive" cumulative patches, a nice change that makes it simple for the DBA to chose to apply "priority" patches.
Please refer
http://www.dba-oracle.com/t_patch_set_update_psu.htm
